The need to protect documents and other media, both in solid form and in liquid form, against fraud has resulted in a wide variety of authentication systems. Cards with a magnetic trace, i.e. a magnetic strip carrying information, are widely used and such cards are frequently the subject of fraudulent endeavors. The code can be ascertained by imprudent use of the card in public or even in the case of private uses where access into communication of the information may make the code available. Modification of the coded information is likewise possible with equipment which can be available to unauthorized individuals. Hence a magnetic strip or trace on a document or card may not be sufficient to assure the authenticity thereof. Furthermore, the magnetic strips hitherto applied to cards tend to be sensitive to mechanical action and can be damaged, thereby preventing the card from communicating its code or from being authenticated by magnetic readers or the like.